This invention relates to light emitting diodes including laser diodes and incoherent LEDs.
In the last decade, significant improvements in three essential components underlie the maturation of optical communications systems from laboratory curiosities to commercial realities. These components are double heterostructure (DH) semiconductor light sources capable of continuous wave (CW) operation at room temperature, glass optical fiber having low optical loss windows at wavelengths matching the source emission, and low noise detectors capable of efficiently converting the transmitted radiation to a corresponding electrical signal.
The low loss windows of commercially available optical fibers typically occur at about 1.1 .mu.m, 1.3 .mu.m, and 1.55 .mu.m where the absorption loss (primarily due to OH in the fiber material) is .ltoreq.1 db/km. Early in the decade, workers recognized the obvious advantages of fiber optic transmission in the longer wavelength windows; and accordingly they fabricated, with varying degrees of success, DH lasers for operation in that range. Their attention was directed primarily to two materials systems:
InP-In.sub.1-x Ga.sub.x As.sub.1-y P-InP and Al.sub.x Ga.sub.1-x As.sub.1-y Sb.sub.y -GaAs.sub.1-y Sb.sub.y -Al.sub.x Ga.sub.1-x As.sub.1-y Sb.sub.y.
The GaAs.sub.1-y Sb.sub.y DH lasers typically contained relatively large amounts of Sb (y.apprxeq.0.12-0.15) to shift the wavelength to .about.1 .mu.m and were step-graded to GaAs substrates to provide stress relief. As summarized by H. C. Casey, Jr. and M. B. Panish, Heterostructure Lasers, Part B, Academic Press (1978) at pages 38-41 and 56-57, respectively, DH lasers in both of these systems have been operated CW at room temperature, but lattice matching problems in the GaAsSb system, and the ability to generate longer wavelengths (e.g., 1.1-1.6 .mu.m) in the InGaAsP system, have combined to render it unlikely that long wavelength GaAsSb lasers grown on GaAs substrates will ever see commercial use.
On the other hand, the easiest materials system from which to make heterostructure light sources, LEDs or lasers, is the Al.sub.x Ga.sub.1-x As system because of the extremely close, albeit not perfect, lattice match between GaAs and AlAs. Unfortunately, Al.sub.x Ga.sub.1-x As sources produce light at wavelengths shorter than about 0.9 .mu.m and thus are constrained to operate at wavelengths below the 1.1 .mu.m window where fiber losses are typically 3 db/km. In fact, Al.sub.x Ga.sub.1-x As-Al.sub.y Ga.sub.1-y As-Al.sub.x Ga.sub.1-x As DH lasers with the best reliability to date have y.about.0.07 in the active layer and thus emit light at .about.0.82 .mu.m in the infrared. These lasers are more than two orders of magnitude more reliable than similar lasers with y=0 (pure GaAs active layer). However, if reliable lasers could be made in the Al.sub.x Ga.sub.1-x As materials system that emit light at, or slightly longer than, the pure binary GaAs wavelength of 0.87 .mu.m, then the practical advantage of the close lattice match would be preserved, and the corresponding fiber loss could be improved from about 3 db/km to 2 db/km. Accordingly, longer fiber optic links and/or longer distances between repeaters in fiber optic systems would be made possible.
On the other hand, when z.gtoreq.0.15 in the active layer of Al.sub.z Ga.sub.1-z As heterostructure light sources, the emission is at visible wavelengths (e.g., .ltoreq.0.78 .mu.m). Emission at such short wavelengths is generally not suitable for present-day fiber optic transmission because the optical absorption would be very high (of the order of 10 db/km). But, such visible wavelength lasers might be useful in other applications, for example video discs, where the shorter wavelength advantageously increases information storage density.